Prepare the Cake Batter
Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking dish to prevent sticking. In a large mixing bowl, combine the white cake mix and pistachio pudding mix. Stir to evenly distribute the pudding throughout the dry cake mix.

Mix the Ingredients
Add the eggs, club soda, and vegetable oil to the bowl. Use an electric mixer to beat the mixture on medium speed for about 2 minutes until smooth and well combined. Fold in the chopped pecans using a spatula to add texture and enhance the flavor.

Bake the Cake
Pour the batter into the prepared baking dish and spread it evenly. Place the dish in the preheated oven and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Once baked, remove from the oven and let it cool completely before frosting.

Prepare the Frosting
In a mixing bowl, whisk together the pistachio pudding mix and cold milk until the mixture thickens. Gently fold in the whipped topping with a spatula to maintain its light, airy texture.

Assemble the Cake
Spread the frosting evenly over the cooled cake. Use a spatula to create smooth layers, and sprinkle chopped pecans on top for added crunch and decoration.

Chill and Serve
Refrigerate the cake for at least 1 hour before serving to allow the flavors to meld. Slice into squares and enjoy!